Temperature alarm utilizing paired positive and negative coefficient thermistors

ABSTRACT

An engine temperature warning device which detects the temperature of an engine by means of a thermosensitive element comprising the combination of a negative characteristic thermistor and a positive characteristic thermistor which are thermosensitive semiconductor resistors to operate a contactless electronic solid state switching circuit to energize and deenergize a warning lamp.

United States Patent Miyamoto et al.

Apr. 17, 1973 TEMPERATURE ALARM UTILIZING PAIRED POSITIVE AND NEGATIVECOEFFICIENT THERMISTORS Inventors: Mamoru Miyamoto, Neyagawa; MinoruTakaichi, Suita, both of Japan Matsushita Electric Industrial Co., Ltd.,Kadoma-shi, Osaka, Japan Filed: Dec. 7, 1970 Appl. No.: 95,635

Assignee:

Foreign Application Priority Data Dec. 11, 1969 Japan ..44/l18195 U.S.Cl ..340/228 R, 73/362, 317/41 Int. Cl. ..G08b 21/00 Field of Search..340/228;

73/362.l362.7, 362 AR, 359; 317/41 [56] References Cited UNITED STATESPATENTS 3,363,466 1/1968 Guidi ..73/362 AR X 3,186,229 6/1965 Liben..73/399 X 2,175,890 10/1939 Glowatzki ..324/106 X 3,444,399 5/1969Jones ..317/41 X Primary Examiner-John W. Caldwell AssistantExaminerWilliam M. Wannisky AtI0rneyStevens, Davis, Miller & Mosher 5 7ABSTRACT An engine temperature warning device which detects thetemperature of an engine by means of a thermosensitive elementcomprising the combination of a negative characteristic thermistor and apositive characteristic thermistor which are thermosensitivesemiconductor resistors to operate a contactless electronic solid stateswitching circuit to energize and deenergize a warning lamp.

1 Claim, 4 Drawing Figures PATENTEnmnm; 3728.702

sum 1 0F 2 RES/STA N05 (11 I o Wald/W TEMPERATURE OR ATTORNEYTEMPERATURE ALARM UTILIZING PAIRED POSITIVE AND NEGATIVE COEFFICIENTTHERMISTORS The present invention relates to an improvement in an enginetemperature warning device.

A conventional engine temperature warning device is such that abimetallic strip provided at its pair of tips with contacts detects theengine temperature to energize an electric lamp by closing the contactsat a predetermined low or high temperature. Since this device has manymechanical parts, it may be damaged by vibrations. Moreover, since thisdevice has contacts provided to the bimetallic switch for energizing anddeenergizing the warning lamp, there is a problem as to its durabilityand chattering occurs. Thus, the conventional engine temperature warningdevice is less reliable than desired.

An object of the present invention is to provide an engine temperaturewarning device which is durable, free from the influence of enginevibrations, free from chattering, accurate in operation, and reliable.

The above object is attained in the present invention by detecting thetemperature of an engine by means of a thermosensitive elementcomprising the combination of thermistors of positive and negativecharacteristics to operate a contactless electronic solid stateswitching circuit to energize and deenergize a warning lamp.

The present invention will become more apparent from the followingdetailed description of the preferred embodiments of the presentinvention when taken in conjunction with the accompanying drawings, inwhich:'

FIG. 1 is a circuit diagram of an embodiment of the engine temperaturewarning device according to the present invention;

FIG. 2 is a resistance versus temperature characteristic of thetemperature detecting part of the embodiment of FIG. 1;

FIG. 3 is a circuit diagram of another embodiment of the enginetemperature warning device according to the present invention; and

FIG. 4 is a resistance versus temperature characteristic of thetemperature detecting part of the embodiment of FIG. 3.

Referring to FIG. 1 a detector 4 comprising a series combination of anegative characteristic thermistor 2 and a positive characteristicthermistor 3, which are thermosensitive semiconductor resistors, detectsthe temperature of an engine to generate a signal which operates aSchmitt circuit 1 comprising preliminary and subsequent stagetransistors 5 and 6, respectively to perform switching. The signal fromthe Schmitt circuit l is amplified by a last stage transistor 7 toenergize a warning lamp 8. The detector 4 is connected in parallel witha variable resistor 10 between a power source 9 and the base of thepreliminary stage transistor 5. The variable resistor 10 adjusts theoperating point of the temperature at which the warning lamp 8 is lit orextinguished. A variable resistor 11 connected between the emitters ofthe transistors 5 and 6 adjusts the hysteresis width of the temperatureat which the lamp 8 is lit or extinguished. The conductive andnon-conductive states of the transistor 6 control those of thetransistor 7 to energize or de-energize the warning lamp 8.

In FIG. 2 the curve A represents a resistance versus temperaturecharacteristic of the negative characteristic thermistor 2, the curve Brepresents a resistance versus temperature characteristic of thepositive characteristic thermistor 3, and the curve C represents aresistance versus temperature characteristic of the detector 4 whichcomprises a series connection of the negative characteristic thermistor2 and the positive characteristic thermistor 3. The curve C is asynthesized one of the curves A and B.

. In operation, when the engine temperature is at a low level and theresistance of the detector 4 is higher than the point a on the curve Cin FIG. 2, the transistor 5 of the Schmitt circuit 1 is non-conductivebecause the base potential of the transistor 5 is lower than a presetoperating point. As a result, the transistor 6 becomes conductive, whichmakes the last stage transistor conductive to energize the warning lamp8. When the temperature of the engine rises and the resistance of thedetector 4 becomes lower than the point b on the curve C, the transistor5 becomes conductive because the base potential of the transistor 5rises. Then the transistor 6 turns off and the transistor 7 also turnsoff to extinguish the warning lamp 8. When the temperature of the enginefurther rises to put the resistance of the resistor 4 at a higher valuethan the point a on the curve C, the base potential of the transistor 5becomes lower than the operating point to make the transistor 5non-conductive. Then the transistor 6 turns on to energize the warninglamp 8 through the transistor 7 being in a conductive state.

In case the temperature of the engine falls from a high level, theoperation of the device is as follows. When the resistance of thedetector 4 becomes lower than the point b on the curve C, the basepotential of the transistor 5 becomes higher than the operating point toput the transistor 5 in a conductive state. The transistors 6 and 7 arethen made non-conductive, and the warning lamp 8 is de-energized. In asimilar manner, the variation in the temperature of the engine isdetected by the detector 4, and the warning lamp 8 is energized orde-energized by the Schmitt circuit 1. As has been stated above, thetemperature at which the warning lamp 8 is energized or de-energized isadjusted by the variable resistor 10, and the latitude of the temperaturof the energization and de-energization of the warning lamp 8 isadjusted by the variable resistor 11.

FIG. 3 shows another embodiment of the present invention similar to theembodiment of FIG. 1 except that the negative characteristic thermistor2 and the positive characteristic thermistor 3 of the detector 4 are inparalled connection in contrast to the series connection in FIG. 1.

The operation of this embodiment is similar to that of the device ofFIG. 1 except that the warning lamp 8 is energized when the resistanceof the detector 4 is lower than the point a' or a on the curve C in FIG.4, which shows similar curves to those in FIG. 2, and the warning lamp 8is de-energized when the resistance of the de tector 4 is higher thanthe point b or b.

In any case, according to the present invention the warning lamp isenergized at temperatures below and above or outside the predeterminedfavorable temperature range.

What is claimed is:

1. An engine temperature warning device comprising a temperaturedetector including a pair of thermistors of positive and negativecharacteristics, a switching circuit consisting of a Schmitt circuitincluding a pair of transistors, the base of the first transistor ofsaid Schmitt circuit being connected to a DC power source through saiddetector, a variable resistor connected in series with said detectoracross said DC power source for setting said temperature range, saidbase of said first transistor being connected to a junction point ofsaid detector and said variable resistor, and another varia-

1. An engine temperature warning device comprising a temperaturedetector including a pair of thermistors of positive and negativecharacteristics, a switching circuit consisting of a Schmitt circuitincluding a pair of transistors, the base of the first transistor ofsaid Schmitt circuit being connected to a DC power source through saiddetector, a variable resistor connected in series with said detectoracross said DC power source for setting said temperature range, saidbase of said first transistor being connected to a junction point ofsaid detector and said variable resistor, and another variable resistorconnected between the emitters of said pair of transistors of saidSchmitt circuit in series therewith for setting the hysteresis width ofthe energization and deenergization of said lamp, an amplifyingtransistor connected to the second transistor of said Schmitt circuitfor amplifying a signal from said Schmitt circuit, and a warning lampconnected to said amplifying transistor, said lamp being energized bythe output of said amplifying transistor at temperatures outside apredetermined temperature range.